1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method in which an electrifying bias applied to electrifying means electrifies a surface of a photosensitive member, an electrostatic latent image is thereafter formed on the surface of the photosensitive member, and a development bias is thereafter applied to developer means so that a toner visualizes the electrostatic latent image into a toner image.
2. Description of the Related Art
This type of an image forming apparatus often sees a change in an image density due to the following factors: fatigue, degradation with age or the like of a photosensitive member and a toner; a change in a temperature, a humidity or the like around the apparatus; and other causes. Noting this, a number of techniques have been proposed which aim at stabilizing an image density through appropriate adjustment of a density control factor such as an electrifying bias, a development bias, a light exposure dose, etc. For example, the invention described in the Japanese Patent Application Laid-Open Gazette No. 10-239924 requires to properly adjust an electrifying bias and a development bias in an effort to stabilize an image density. That is, according to this conventional technique, reference patch images are formed on a photosensitive member while changing an electrifying bias and/or a development bias and an image density of each reference patch is detected. An optimal electrifying bias and an optimal development bias are thereafter determined based on the detected image densities, and a density of a toner image is accordingly adjusted.
However, the conventional technique described above requires to identify an electrifying bias/development bias characteristic before forming reference patch images, and to set an electrifying bias and a development bias for creation of reference patch images, such that the characteristic is satisfied. In order to stabilize an image density based on a calculated optimal electrifying bias and development bias, it is necessary to identify an electrifying bias/development bias characteristic of each image forming apparatus, which is troublesome.
Further, an electrifying bias/development bias characteristic does not always stay constant but may change with time. If the characteristic changes, it is difficult to accurately calculate an optimal electrifying bias or an optimal development bias. While appropriate updating of the electrifying bias/development bias characteristic solves this problem, the updating is bothersome and disadvantageous in terms of maintainability.
Meanwhile, other technique for stabilizing an image density is the invention described in Japanese Patent Application Laid-Open Gazette No. 9-50155. According to the described invention, a reference patch image, which is a patch image obtained by outputting groups of three-dot lines for every three dots, is formed on a photosensitive drum, and a sensor reads patch images thus created, whereby a line width is detected. A laser power is controlled based on the detected line width, a light exposure dose is accordingly adjusted so that a desired line width will be obtained, and an ideal line image is obtained.
However, a line image is basically a one-dot line which is drawn with one laser beam, and therefore, simply controlling a line width of a multi-dot line as in the conventional technique can not realize a precise adjustment of a line image.
A main object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density at a high accuracy in a simple manner.
Other object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize an image density of a line image.
In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image based on image densities of a plurality of patch images.
According to a first aspect of the present invention, control means performs a development bias calculation and an electrifying bias calculation in this order. In the development bias calculation, after sequentially forming a plurality of toner images as first patch images while changing the development bias, densities of the first patch images are detected, and an optimal development bias, which is necessary to obtain the target density, is determined based on the densities of the first patch images. In the electrifying bias calculation, after sequentially forming a plurality of toner images as second patch images while changing the electrifying bias with the development bias fixed to the optimal development bias, densities of the second patch images are detected, and an optimal electrifying bias, which is necessary to obtain the target density, is determined based on the densities of the second patch images. Thus, it is possible to obtain an optimal electrifying bias and an optimal development bias without using an electrifying bias/development bias characteristic.
According to a second aspect of the present invention, a plurality of patch images, each of which is formed by a plurality of one-dot lines that are apart from each other, are formed on a photosensitive member or a transfer medium. Control means adjusts an image density of a toner image based on the image density of the patch images. Hence, it is possible to stabilize an image density of not only a line image which is formed by a P-dot (Pxe2x89xa72) line but of a line image which is formed by a one-dot line, to thereby stably form a fine image with an appropriate image density.